This invention relates to an arc detector for a gas insulated electrical apparatus, such as a bus bar insulated with SF.sub.6 gas.
FIG. 1 shows a conventional arc detector as described in Japanese patent application publication No. 58-115370, wherein an insulating gas such as SF.sub.6 fills a metal sheath or tube 1 in which a bus bar 5 is supported by spaced insulating members 51. An arc detector 2 is mounted to an aperture in the tube. When an accidental arc 4 is produced between the tube 1 and the bus bar 5, the high pressure generated at the arcing point is transmitted to the detector 2, shown in detail in FIG. 2. As may be seen therein, a movable contact 61 of a micro switch 7 engages a stationary contact 62 when a bellows 6 is expanded by the arc pressure in the tube, and the detected arc condition is signalled by an indicator 3.
With such a conventional arc detector it is difficult to accurately regulate the sensing or triggering level to accommodate different geometries and volumes of the apparatus being monitored, and to reliably respond to small pressure deviations as produced by a very weak arc.